1. Field of the Invention
The present invention relates to a coaxial high-frequency cable.
The invention also concerns a dielectric material for use in a cable.
The invention can be utilized in the transfer of a radio-frequency signal, whether digital or analog, when the signal transfer system requires a low attenuation over the trans-mission path. Typically, such an application is in the high-power transmission from the power amplifier stage of a radio transmitter to the radiating antenna element proper or connection of a receiving antenna to the input stage of a radio receiver, or a combination of similar signal paths. An example of such an application is found at the base stations of cellular phone networks. Another application is in the radio-shadow areas of said cellular phone systems such as tunnels, cellars, etc., where this type of cable can be used as the radiating element when provided with a perforated leaky outer conductor. Also in cable-TV networks in which the transmitted signal conveys both analog and digital television pictures, the cable according to the invention is useful, as well as on the subscriber lines of modern telephone systems (access networks) which use a coaxial cable as the transmission medium in the transfer of wideband information. Furthermore, the invention is useful in symmetrical cabling of a wideband data network. The benefits of the invention are the higher the wider the required transmission bandwidth, typically ranging from a few megahertz to a few gigahertz.
2. Description of the Background Art
Cable structures of both coaxial and symmetrical construction suitable for high-frequency transmission have been made in the art with a polymer dielectric as soon as polyolefin polymers of suitable qualities appeared on the market in the 1940's. In order to achieve a low permittivity (.epsilon..sub.r) and dissipation factor (tan .delta.), a countless number of polymer-air dielectric material combinations have been tested over times in order to maximize the fraction of air in the dielectric with the goal of minimizing the attenuation constant of the cable without compromising the mechanical handling properties of the cable. As rule of thumb, the mechanical bending endurance, compression resistance and other durability-related properties are deteriorated when the volume of the solid dielectric material is reduced and replaced by a gaseous medium, whereby the attenuation and dissipation factor of the cable are decreased. A good compromise has been found in an expanded polymer dielectric, conventionally polyethylene, which is formed by foaming from an initially solid polymer dielectric material in an extruder during the cable insulation process.
In early attempts, the foaming step was implemented by compounding the polymer raw material with a specific chemical foaming agent which was capable of blowing closed cells of desired size in the polymer dielectric. A problem of this approach is that the polymer dielectric material traps residues of the foaming agent that deteriorate the dissipation factor and attenuation at the upper end of the frequency range. Partially with the goal to overcome this drawback, physical foaming methods were developed based on injecting into the extrusion process some inert gas, originally fluorocarbon gas but later nitrogen or carbon dioxide, in order to blow the gas-filled expanded cells. Practical experience has, however, shown that both of these prior-art foaming methods will at some state reach certain ultimate limits of attenuation and dissipation factor that cannot be exceeded, because the foaming ratio cannot be passed further due to the deterioration of mechanical properties and because the basic qualities of available polymer grades, which determine the achievable electrical properties, are already maximally exploited.